The invention relates to a process and an apparatus which prevents a motor vehicle, generally used in vehicular traffic and provided with an electrohydraulically controlled automatic transmission, from being shifted into a running gear before requirements of safe shifting are met. The invention also assures that the lowest fuel consumption can be achieved, if the motor vehicle is waiting with a running motor.
With certain motor vehicles provided with automatic transmissions, it is possible to switch the shift controller into position to obtain the required gear change, quite independently of the actual running state of the motor vehicle and the number of revolutions of the motor. If the driver brakes the vehicle while shifting into a running gear, accidents can be avoided by overcoming the dragging force of the hydrodynamic torque converter. However, this involves energy losses.
Up to now the problem has been dealt with by connecting the hydraulic lines for actuating the clutches of the starting stages (i.e., low, reverse) of the automatic transmission, with the return via separate lines, each with an electromagnetic valve. Such an arrangement is specified, e.g., in the GB-PS No. 2 126 291. With this solution, a prerequisite of closing said electromagnetic valve is that requirements of safe start-up shifting should be met. The drawback of the solution, however, is that practically a second actuating and controlling system must be provided for the starting stages of the transmission.
The aim of the invention is to develop a control system for the automatic transmission, which is able to prevent start-up shifting if the prerequisites of safe shifting are not established. That means that the second control system, mentioned above, is not required. According to the invention, the objective can be realized by providing signals, for controlling the electromagnetic valves for switching the start-up stages which are influenced by the conditions of safe start-up shifting.
A further objective of the invention is that during waiting time the motor vehicle should consume the minimum quantity of fuel. This requirement can be met so, that with the gas pedal released, and with a very small decelerating motion of the vehicle, the automatic transmission is switched to neutral.
When realizing the process according to the invention, the proceeding takes place in the following manner:
Sensors are provided for detecting (a) the motion of the vehicle, (b) the speed of the engine, and (c) the position of the throttle (fuel pedal). When, in combination, vehicle speed is near zero, engine speed is near idle speed, and the throttle is released, a first shift-enabling signal is generated. Thereafter, by means of the gear selecting switch, a control signal is produced directing the switch of the automatic transmission into low speed or reverse gear, as selected by the user. Thereafter, by displacing the accelerating pedal from its "basic" or idle position, a command signal is transmitted when the enabling signal, the control signal and the command signal exist in combination, the switch signal is formed, by which the electrohydraulic valves are controlled to the desired switching position, thus shifting to the required gear of the automatic transmission.
With a preferred mode of realization of the process according to the invention, a signal is generated characterizing the direction of motion of the motor vehicle. By using said signal, the direction of motion of the motor vehicle is compared in the control unit with the starting direction selected by the gear selecting switch. If the direction of vehicle movement is in compliance with the direction of the selected start-up gear, the signal expressing near zero velocity of the vehicle is completed.
With another preferred mode of realization of the process according to the invention, when there are signals indicating the "idle" (released) position of the accelerator pedal, and indicating also the slowing motion of the motor vehicle to below a predetermined low level of velocity, the shift switching signal is interrupted. All stages of the automatic transmission are disconnected and the transmission returns to "neutral", so the vehicle can coast freely. Thereafter, by merely displacing the accelerator pedal from its basic ("idle") position, the automatic transmission is reactivated into the previously selected shift condition.